The mechanisms of the reactions of thrombin and collagen with platelets will be investigated. A detailed description of the kinetics of the reactions will be attempted by measuring release of Ca2 plus and ATP and by the use of fluorescent membrane probes. The reaction with thrombin will be compared with those of trypsin and papain. The binding of thrombin to platelets will be studied using inhibited, chemically modified, or radioactive labelled thrombin. The knowledge obtained will be used to develop platelet function tests for use in studying the mechanisms of drugs that affect platelet function.